This invention is directed toward a method of enhancing oil recovery and more particularly enhancing oil recovery by stimulating resident microorganisms through the injection of specific nutrient formulations in combination with a water alternating gas injection process. More specifically, and without limitation, this invention relates to supplementing the immiscible water injection cycle with nutrients during a water alternating gas injection process to improve oil and oil residue release from oil-containing rock formations.
Water Alternating Gas (WAG) is well known in the art and has been applied to improve oil recovery since the late 1950's. The Schlumberger Oilfield Glossary defines “Water Alternating Gas” as “An enhanced oil recovery process whereby water injection is carried out alternately for periods of time to provide better sweep efficiency and reduce gas channeling from injector to producer.” This process was originally intended to improve sweep efficiency during gas flooding in an oil-containing reservoir but variations of the WAG technique include injecting gas as a supplement to water injection and vice versa for the purpose of improving sweep efficiency in the oil-containing reservoir.
The WAG technique is not singular, but rather is a combination of two oil recovery processes: gas injection and waterflood. Miscible injections result in homogeneous mixtures while immiscible injections result in heterogeneous mixtures—here, water, gas, and “swollen” oil. Particularly, in the immiscible water injection process, the portion of the injected gas dissolved in the oil reduces the oil viscosity. In addition to reducing viscosity, the dissolved gas also swells the oil, so for a given fixed residual oil saturation, less oil remains after an immiscible waterflood cycle. These two mechanisms have been demonstrated by numerous laboratory PVT and coreflood tests and are well known in the art. The gas injection cycle may be natural gas, carbon dioxide or nitrogen depending on the availability of the gases and the relative economics of the particular application.
As such, the WAG injection process aims to produce more oil out of reservoirs and the effectiveness of WAG is well known in the art. Injection of gas slugs alternated with water slugs is the common practice presently used for controlling gas mobility. This process, however, is not free from deficiencies. It is well known that remaining (residual) oil in flooded rock may be lowest when three phases—oil, water and gas—have been achieved in the rock formation's pore volume.
“Three-phase Relative Permeability” describes the complex relationships of fluid flow in porous media containing the three physical phases of fluids; i.e., oil, gas and water. Three-phase flow is a very complex relationship and has been studied in detail by numerous researchers. A problem with gas injection, both miscible and immiscible, is the inherently unfavorable mobility ratio and the resulting poor volumetric sweep in reservoirs. Here, the three-phase flow relationship may be generally described by the schematic of constant immobile saturations for each fluid phase within the pore space shown in FIG. 1.
Accordingly, a unique and innovative technique in which nutrient materials are supplemented into the injected water during the immiscible water injection cycles of the WAG process in order to recover high percentages of oil and oil residue is needed. Nutrient supplementation uniquely introduces an additional oil release mechanism to farther enhance the WAG process. This process provides an additional dimension to the WAG process by focusing not only on displacement efficiency but also on a unique oil release mechanism. Needed is a process that reduces residual oil saturation through the supplementation of the immiscible water injection cycle in the WAG process to release additional oil and alter the three-phase relative permeability and improve overall oil flow rate and ultimate recovery.
Thus, it is a primary objective of this invention to provide a method of microbially assisted WAG injection as a means of enhanced oil recovery by supplementing the immiscible water injection cycle with nutrients to improve oil and oil residue release in oil-containing rock formations that improves upon the art.
Another objective of the present invention is to provide a method of improving oil recovery in oil containing rock formations.
Yet another objective of the present invention is to provide a method of clearing residual oil from oil-containing rock formations.
Another objective of the present invention is to provide a method of creating an oil swell within the oil-containing rock formations.
These and other objectives, features, and advantages of the invention will become apparent from the specification and claims.